The ability to avoid infection depends on animal mechanisms of innate immunity. The innate immune system is responsible for the primary response and one type of granular cells, mast cells (MCs) has been reported to play a critical role as part of the defence function against pathogens and evidence for its involvement in the immune system of fish is growing. Antimicrobial peptides (AMPs) are widely distributed in nature and are considered to be ancestral components in the evolution on innate immunity. Several types of antimicrobial peptides (AMPs) have been isolated from fish and they are present in mucosal surfaces and in immune cells such as MCs. One type of AMPs expressed by fish MCs is piscidins. Twenty-four perch, Perca fluviatilis (22,95 ± 7,48, mean total length ± S.D.) from Lake Piediluco (Centre Italy) were caught by professional fishermen using a gill net. Fourteen pike were infected with an intestinal parasite, Acanthocephalus lucii. The pathological changes and the immune response induced by worms were assessed using histological, immunohistochemical and ultrastructural techniques. The parasites penetrate not deeply into the intestinal wall and induce slight inflammation to the organ. The main damage caused by worms to the host digestive tract was the destruction of the mucosal epithelium covering the villi next to the parasites site of attachment, which included necrosis and degeneration. Immunostaining of the perch intestine with antibodies against the peptide antibiotics piscidin 3 (anti-HAGR) and piscidin 4 (anti-5.3-02-3A) showed subpopulations of MCs that were positive. Piscidin positive MCs were observed especially among the epithelial cells of the intestine but also within the submucosal layer. In both uninfected and infected perch, the number of MCs positive to piscidin 4 was higher than those immunoreactive with piscidin 3 (ANOVA, p < 0.05). For both piscidins, there was no significant difference in the number of positive MCs between infected and uninfected intestine (ANOVA, p > 0.05). The immunostaining intensity was also compared in infected and healthy fish but no distinct differences were noticed (ANOVA, p > 0.05). Our study provides the first evidence of the presence of piscidins in digestive tract of a fish infected with intestinal helminths
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